A fuel cell has a cathode, an electrolyte, and an anode. An oxidizing agent, e.g. air, is supplied to the cathode, and a fuel, e.g. water, is supplied to the anode.
Different types of fuel cells are known, for instance the SOFC fuel cell from publication DE 44 30 958 C1 and the PEM fuel cell from publication DE 195 31 852 C1 [CA 2,240,270].
The SOFC fuel cell is also called a high-temperature fuel cell because its operating temperature can be up to 1000° C. In the presence of the oxidizing agent, oxygen ions form on the cathode of a high-temperature fuel cell. The oxygen ions diffuse through the electrolytes and recombine on the anode side, creating water with the hydrogen that comes from the fuel. As this recombination occurs electrons are released and thus electrical energy is generated.
As a rule, a plurality of fuel cells are generally joined together, electrically and mechanically, by connecting elements, also called interconnectors, for attaining great electrical outputs. Electrically series-switched fuel cells that are stacked upon one another result from interconnectors. This arrangement is called a fuel cell stack. Fuel cell stacks comprise interconnectors and the electrode/electrolyte units.
In addition to the electrical and mechanical properties, interconnectors normally also possess gas-distributing structures. This is realized using bars and grooves (DE 44 10 711 C1) [U.S. Pat. No. 5,733,682]. Gas-distributing structures cause the fuel to be distributed uniformly in the electrode spaces (spaces in which the electrodes are located).
The following problems can disadvantageously occur with fuel cells and fuel cell stacks:                Metallic interconnectors with a high aluminum content form Al2O3 coatings that disadvantageously act like an electric insulator.        Given cyclical temperature loads, generally thermal stresses occur that are associated with movements of the individual parts relative to one another; these result from the different expansion behaviors or the different expansion coefficients of the materials used during operation.        
In this regard, in the prior art there is still not adequate compatibility between the comparatively high expansion coefficients e.g. of the metallic interconnectors and the currently known electrode materials, whose expansion coefficients are comparatively low. On the one hand, thermal stresses can occur between electrodes and interconnectors. These can result in destruction within the fuel cell. However, on the other hand this also applies to glass solders that are frequently used in fuel cells, the glass solders being intended to ensure that the fuel cells are sealed. The interconnectors known from the prior art are made of metal, providing good electrical conductivity. However, one disadvantage of the metal interconnectors is comprised in that they are susceptible to corrosion and the service life of the fuel cell is shortened by this. In particular, the use of ferritic chromium steel (e.g. Crofer 22, a steel alloy having 22% chromium) represents a problem for the cathode of the fuel cell. At high temperatures, this material forms a chromium oxide protective layer that is sufficiently conductive. However, under operating conditions chromium evaporates continuously from this protective layer and deactivates the active centers of the cathodes of the fuel cell, so-called chromium poisoning. This means a continuous drop in the performance of the fuel cell. Known from DE 195 47 699 is a selectively coated interconnector that comprises a chromium oxide-forming alloy. It has a protective layer in the area of the gas guiding surfaces that reduces the effects of corrosion and that is an electrical insulator, e.g. a thin Al2O3 layer. In addition, the interconnector is coated with a mixed oxide layer on the electrode contact surface, leading to an increase in conductivity and to a reduction in the rate of evaporation. This mixed oxide layer is attained e.g. by applying a thin layer made of a metal or metal oxides that forms a mixed oxide (e.g. Spinell type) when used at high temperatures with Cr and/or Cr2O3 on the oxide/gas limiting surface. Suggested as suitable metals or their oxides are Fe, Ni, or Co, which modify the physical properties of the Cr2O3 in the desired manner. However, these layers are only stable under certain conditions and tend to burst or crack. Another disadvantage is that the production method for these thin layers is complex.